Reflected light from the glossy surface of a substance comprises regularly-reflected light from a light source in the form of an image of the light source, and diffusedly reflected light which is relected in diffused form due to the specific state of the surface of the substance. Diagrams indicating the quantities of this reflected light are shown in FIGS. 1 and 2. Illuminant light 1 from a light source (not shown) is reflected from a reflecting plane 2 and turns into regularly-reflected light 3 and diffusedly-reflected light 4. The regularly-reflected light is reflected in the direction of the regular reflection angle which is equal to the incident angle .theta., while the diffusedly-reflected light is reflected around the regularly reflected light. FIG. 1 shows the state in which the central region of the reflected light is narrow and sharply defined and of relatively high intensity, and the intensity of the diffusedly-reflected light is relatively low since the regular reflection is strong. FIG. 2 shows the state in which the central region of the reflected light is broad and not well defined and of relatively low intensity, and the diffusedly-reflected light is large since the regular reflection is weak. Since every light source has a specific size, the illuminant light 1 is a bundle of rays having specific dimensions; consequently, the regularly-reflected light therefrom also occupies a regularly-reflected light area 3 of specific dimensions, while the diffusedly-reflected light area 4 is located around the area 3.
For describing the reflection characteristics of this glossy surface of a substance, the terms "gloss" and "glossiness" or "luster" (for a feeling of luster) are used. One prior art method of measuring the degree of the gloss is the specular gloss measuring method (JIS Z8741 Method of Measurement for Glossiness). FIG. 3 shows the structure of a specular gloss meter used in this method. Light from a light source in the form of a lamp 5 passes through a lens 6, is focussed thereby on a slit 7, and is further turned into parallel rays by a lens 8. These light rays are reflected from the surface 2 of a substance the gloss of which is to be measured, and are focussed on a receiver stop 10 by a lens 9, and then reach a photoelectric cell 12 of a receiver 11. The light received by said photoelectric cell 12 is that condensed in the stop 10 by the lens 9 in this structure. Therefore, the light passing through the stop 10 is only regularly-reflected light containing light moving parallel to an optical axis in the direction of regular reflection, or light deviating slightly therefrom, and thus diffusedly-reflected light in the vicinity of the regularly-reflected light which intersects the optical axis of the regular reflection or is divergent therefrom does not strike the aperture of the stop and thus is not received by the photoelectric cell. Consequently, the cell receives only light regularly or directly reflected from the light source. FIG. 4 shows the cross-section of the direct light received by the cell. Said cross-section is composed of the central portion 14 of the illuminant light having the same size as the image 13 of an illuminant filament, and regularly-reflected light 15 surrounding said portion. The specular gloss meter thus constructed is generally used widely for evaluating gloss of the surface of a substance.
However the specular gloss measured thereby does not correlate well with a visual estimation of gloss. At the present neither a gloss measuring method nor a gloss meter which correlates well with the result of visual measurement has been developed.
In view of the actual state of this prior art of measuring gloss, the present inventors further studied the reason why gloss measured with the conventional gloss meter, namely the specular gloss meter, does not correlate well with that estimated visually.
Generally, the intensity of regularly-reflected light obtained from the reflection of illuminant light on a specular plane is called "brightness", while the intensity of reflected light in the vicinity of the regularly-reflected light, which comprises the reflection from the surface of a substance and the internal reflection in a surface coating layer, is called the "specific luster" attributed to the surface of the substance. In the case of a smooth plane on which the brightness is high while the specific luster is low, the human eye is apt to sense the high brightness as glare, while being unable to preceive specific luster. Hence, the eye makes a comparison on the basis of the intensity of the glare. Therefore, a high degree of correlation exists between specular gloss and visual estimation in this case.
In the case of a surface of a substance which is not so smooth, the human eye tries instinctively to preceive the specific luster of the substance, keeping away glare caused by brightness. Hence, specular gloss measured by receiving brightness of direct light from a light source and regularly-reflected light around said direct light together has a poor correlation with the visual estimation of that gloss. Moreover, in the case of a surface close to a matte surface in texture, on which brightness is very low, the ratio of the specific luster of the surface of the substance to the brightness is relatively large. Hence, correlation between measured specular gloss and visual estimation of gloss is relatively high.
For these reasons, the incident angle of the illuminant light is changed in accordance with the smoothness of the surface of the substance to be tested when measurement is conducted according to the above-described specular gloss measuring method. In the method of JISZ8741, it is provided that the measurement is to be conducted with the incident angle set at 20 degrees when specular gloss is comparatively high, at 45 or 60 degrees when it is intermediate, and at 75 to 85 degrees when it is low. Even though measurement is conducted for 20 degree gloss, 60 degree gloss or other angle gloss by varying the incident angle in this way, gloss values obtained by this measurement do not correlate well with visual estimation of gloss.
Based on the results of the above-described studies, the present inventors concluded that specular gloss did not correlate well with visual estimation of gloss in the prior art because said gloss is regarded thereby as involving the central region of the regularly-reflected light, and thus the discrepency between specular gloss and visually estimated gloss cannot be eliminated even with the cumbersome method in which 20 degree gloss, 60 degree gloss and other gloss must be measured separately. The present inventors have confirmed that a gloss measuring method eliminating said disadvantage of the prior art can be provided according to the present invention.